What Are The “Area Files” That Are Produced With The Mosaics?

Montage returns with each mosaic an “area file” in FITS format that contains the total amount of sky area (steradians) that contributed to the value of a pixel. It weights  the output flux by the “areas” of the contributing inputs.

The Montage Reprojection algorithm, including accurate calculations of the areas of individual pixels contributing to the outoput flux of each pixel.

The Montage Reprojection algorithm, including accurate calculations of the areas of individual pixels contributing to the outoput flux of each pixel.

The file is a useful quality assurance tool and comes into play in three places:

  • Around the edges of a reprojected image, where there may be a pixel where only a tiny  area contributes to its value;
  • When the projection covers enough sky that the projection distortions onto the image plane return different areas for pixels on the edges versus in the middle; and
  • When are co-adding values from multiple images where the previous two effects require different weightings for the contributing pixels.

A concrete example will help illustrate how the values are calculated .  Suppose you reproject image A and an edge pixel in the input has a value of 1.523 but only overlaps the corresponding output pixel by 10%.The output pixel flux value would still be 1.523 but
the relative area value would only be 0.1.  A second image B covers the same output pixel with a value of 1.621 but completely, so the area is 1.0.  Then when we coadd, we want to give the coadded pixel the  value of

(1.523*0.1 + 1.621*1.0) / (0.1 + 1.00) = 1.612

i.e., weighting the output flux by the “areas” of the contributing inputs.  The “area” of the output pixel (which is now more of a weight for future use) is the combined 1.1.  If I have lots of input images for the same output location, I can easily have fairly
large “areas”.

Adapted from material prepared by Dr. John Good (IPAC)

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